The present invention relates to an sintered compact of indium oxide system, and an transparent conductive film of indium oxide system.
As for a transparent conductive film, indium oxide doped with tin (hereinafter referred to as “ITO”) is being widely used as an electrode material of FPD (flat-panel displays) and the like due to its superior characteristics including low resistivity and high transmittance.
There are various uses of a transparent conductor in addition to use in a flat-panel display, but among such uses, demands as a material for window-layer electrode on the optical incidence plane side of solar cells have increased in recent years.
Since the spectral sensitivity of solar cells is up to approximately 1200 nm with crystal silicon type solar cells and up to approximately 1300 nm with CIGS (Cu—In—Ga—Se-based) type solar cells, high transmittance is demanded even up to the foregoing long wavelength range. Moreover, with amorphous silicon solar cells, since the spectral sensitivity is in a short wavelength up to approximately 300 nm, transmittance of the transparent conductive film is demanded to be high also up to the short wavelength range.
Nevertheless, when ITO is used as the material for window-layer electrode of solar cells, ITO is advantageous since it has low resistivity, but on the other hand, there is a problem in that the conversion efficiency of the solar cells will deteriorate, since its transmittance in a long wavelength range more than the vicinity of a wavelength of 1200 nm is inferior because of its high carrier concentration and the long wavelength range of solar light cannot be utilized effectively.
As a transparent conductive film other than ITO, known is a type in which indium oxide is doped with zinc oxide. Although indium oxide doped with zinc oxide enables to obtain a relatively favorable film on a non-heated substrate, since it contains zinc, there are problems in that its moisture resistance is inferior, its long-term stability is insufficient, and its short wavelength transmittance is low.
Under the foregoing circumstances, the present invention focused on indium oxide doped with niobium (hereinafter referred to as “INbO”) as a potential material having high transmittance and low resistivity in both the short wavelength and long wavelength ranges.
The following documents have previously reported on INbO.
Patent Document 1 describes that a low-resistance transparent conductive film can be obtained by adding niobium or the like to In2O3. Nevertheless, it is important for a sintered compact made from these materials to possess characteristics that are required as a sputtering target, but there is no description regarding the target characteristics.
Moreover, Patent Document 1 briefly describes the electrical and optical characteristics of the film that is obtained through sputter deposition. Nevertheless, Patent Document 1 fails to disclose the carrier concentration, and the transmittance at the short wavelength and long wavelength ranges other than a wavelength of 550 nm.
In addition, the deposition condition where the substrate temperature is 300° C. is unacceptable level of high temperature as a condition of a normal production process of transparent conductive films for solar cells. Patent Document 1 additionally shows that a mixture gas, in which a ratio of oxygen to argon is 1:9 and which is of a high oxygen concentration considerably different from the standard condition, is used as a sputter gas.
Patent Document 2 describes In2O3 doped with niobium. Nevertheless, the specification describes that, when In2O3 is doped with niobium, it is extremely effective to concurrently use a tin component in order to achieve low resistance of the obtained product. Moreover, the Examples do not illustrate any instance where niobium is independently added to In2O3, and tin is always added in cases where niobium is added; and further describes that tin should be added at a high concentration of at least 3.5 wt % or higher.
Accordingly, Patent Document 2 fails to specifically describe independently adding niobium to In2O3, and, even in cases of concurrently adding tin, the addition of tin is not in trace amounts.
Moreover, with Patent Document 2, the sputter gas is pure argon and oxygen is not added. Thus, it could be assumed that a part of the oxides configuring the film will be reduced and tend to become a metal component, and it causes the transmittance to deteriorate. Nevertheless, the only description regarding the transmittance of the obtained film is regarding the result at a wavelength of 550 nm, and the transmittance in the short wavelength and long wavelength ranges is unknown. Moreover, the substrate temperature is also a high temperature at 350° C.
Patent Document 3 describes conductive oxide particles having a crystal structure of indium oxide consisting of indium atoms, antimony atoms and oxygen atoms, or to which zinc atoms are added; and further describes about use of niobium in substitute for antimony. The molar ratio of Nb/In is described as being within a range of 0.01 to 0.10.
Nevertheless, Patent Document 3 relates to a deposition method based on particulate coating, and this technology is unrelated to the sintered compact for use as a target of the present invention described later and the transparent conductive film obtained by sputter deposition using such sintered compact.
Furthermore, there is no specific example of conductive oxide particles in which niobium is independently added to indium, but in Table 2 of Patent Document 3, there are Examples in which antimony and niobium are simultaneously added. Nevertheless, with the Examples described in Table 2, the lowest specific resistance value is 3.1 Ωcm. There is no choice but to say that this value is extremely insufficient in order to achieve low specific resistance. This is considered to be a result of the additive amount of niobium, which is used in substitute for antimony, being small.
It is obvious that Patent Document 3 is different from the present invention described later. Only for reference, it has been described herein.
Patent Document 4 describes that a high-density sintered compact is produced by using indium oxide powder as the main component, adding tungsten oxide powder thereto and sintering the mixed powder; and further describes that silicon, titanium, zinc, gallium, germanium, niobium, molybdenum, ruthenium, or tin can be used in substitute for tungsten oxide. Nevertheless, most of the Examples are preoccupied with the addition of tungsten oxide, and there is only one specific example of a substitute element.
This example does not clearly describe about the additive amount in the case of single additive, and the additive amount in the case of plural additives. Moreover, both cases aim to increase the density, and the specific resistance is completely unknown. The listed substitute elements lack materiality, and it cannot be said that Patent Document 4 is technology worth disclosure as a target comprising substitute elements for a transparent conductor.
Non-Patent Document 1 briefly describes the substrate temperature upon depositing In2O3 doped with niobium by the PLD (pulsed laser deposition) method and the electrical and optical characteristics of the film. Nevertheless, it is described that, when the substrate temperature is low, the resistivity is extremely high and the carrier concentration also increases. Moreover, when the substrate temperature is 200° C. or higher, the carrier concentration is an extremely high value in the 1021 cm−3 range, and it is assumed that the transmittance in the long wavelength range is extremely low. However, only the measurement results up to a wavelength of 1100 nm are described. Moreover, it is described that the transmittance is low in a short wavelength, and becomes even lower when the substrate temperature is low.
Non-Patent Document 2 briefly describes the oxygen concentration upon depositing In2O3 doped with niobium by the PLD method and the electrical and optical characteristics of the film. Nevertheless, all results are based on the condition where the substrate temperature is extremely high at 400° C., and, since the carrier concentration is also high when the resistivity is low, it is assumed that the transmittance in the long wavelength range will be low. However, only the measurement results up to a wavelength of 900 nm are described.    [Patent Document 1] Japanese Laid-Open Patent Publication No. H2-309511    [Patent Document 2] Japanese Laid-Open Patent Publication No. H3-15107    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2002-274848    [Patent Document 4] Japanese Laid-Open Patent Publication No. 2006-22373    [Non-Patent Document 1] Journal of Crystal Growth 310 (2008) 4336-4339    [Non-Patent Document 2] Materials Chemistry and Physics 112 (2008) 136-139